The aim of the project is to find physiological correlates to account for the loss of cochlear sensitivity during the initial stages of noise overexposure. Anesthetised guinea pigs are exposed to broad band or octave band continuous noise. During exposure, cochlear function is assessed by the measurement of endocochlear potential, tone-induced cochlear responses and the Na ion, K ion and Cl ion activities of endolymph and perilymph. Results indicate that the primary site of noise damage is at the level of the sensory hair cells of the organ of Corti. An apparent reduction of the permeability of the endolymph/perilymph barrier to potassium ions is correlated with the suppression of hair cell responses during exposure to intermediate levels of noise. After prolonged exposure to high noise levels, the endocochlear potential and endolymph potassium concentration fall presumably as a result of deterioration of the endolymph/perilymph barrier.